1. Field of the Invention
The present invention relates generally to grease compositions designed for use in heavy-duty, high temperature applications such as the lubricating compositions which are used to lubricate journal bearing and rolling element type rock bits used to drill hot subterranean formations.
2. Description of the Prior Art
There is a continuing need to develop functional fluids capable of serving as lubricant compositions in extreme temperature and pressure environments. Such an example environment is that of bits used to drill subterranean formations.
Rock bits of the rolling element and journal bearing types are employed for drilling such subterranean formations in order to produce oil, gas, geothermal steam and other fluids. Such bits have a body with a threaded upper extent which is connected within a drill string leading to the surface and have several, typically three, cutter cones which are mounted on pins integral with the body of the bit at its lower end.
In use, the drill string and bit body are rotated within the borehole and each cone is caused to rotate on its respective pin as the cone contacts the bottom of the borehole to disintegrate earthen formations. As the rock bit begins to penetrate hard, tough earthen formations, high pressures and temperatures are encountered. Typical drilling operations thus take place in an abrasive atmosphere of drilling mud and rock particles which are thousands of feet from the engineer or supervisor, who does not typically have the benefit of oil pressure gauges or temperature sensors at the surfaces to be lubricated.
Lubricants used in the bearing regions of such rock bits are thus a critical element of the life of the rock bit. The grease utilized to lubricate a rock bit of this type will often encounter temperatures above 300.degree. F., thereby subjecting the lubrication system to severe and demanding constraints. The lubricant must not break down under the temperature and pressure conditions encountered, must not generate substantial internal pressures in the bit, must enable flow through passages to the surfaces to be lubricated and must prevent solid lubricant particles from settling out.
Failure of the lubrication system quickly results in failure of the rock bit as a whole. When the rock bit wears out or fails as the borehole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become greater and greater. A successful grease should have a useful life longer than other elements of the rock bit so that premature failures of bearings do not unduly limit drilling.
A variety of grease compositions have been employed in rock bits in the past. Such grease compositions typically comprise a high viscosity, refined petroleum (hydrocarbon) oil or mineral oil which provides the basic lubricity of the composition and may constitute about 3/4 of the total grease composition. The refined hydrocarbon or mineral oil is typically thickened with a metal soap or metal complex soap, the metals being typically selected from aluminum, barium, calcium, lithium, sodium or strontium. Complex, thickened greases are well known in the art and are discussed, for example, in Encyclopedia of Chemical Technology, Kirk-Othmer, Second Edition, A. Standen, Editor, Interscience Publishers, John Wiley and Sons, Inc., New York, N.Y., 1967, pages 582-587. See also Modern Lubricating Greases, by C. J. Boner, Scientific Publications (GB) Limited, Chapter 4.
The prior art shows solid extreme pressure (EP) additives which have been employed to attempt to enhance the lubrication properties of oils and greases. For example, molybdenum disulfide has been used in a wide variety of lubricants as discussed in U.S. Pat. Nos. 3,062,741; 3,170,878; 3,281,355; and 3,384,582. Other solid additives which are widely used include copper, lead and graphite.
It is also known to include metallic oxides like zinc oxide in lubrication oils. U.S. Pat. No. 2,736,700 describes the use of molybdenum disulfide and a metallic oxide such as fumed lead oxide and zinc oxide in a paint-on composition or bonded lubricant. Such bonded lubricants are used for drawing tough metals such as uranium, thorium, zinc and titanium. Such bonded lubricants are inadequate and could not be used in the extreme wear, heavily loaded applications for which this invention is intended.
U.S. Pat. No. 3,935,114, assigned to the assignee of the present invention, teaches the use of molybdenum disulfide and antimony trioxide in a lubricating grease for a journal bearing used in a drill bit. This grease has proved particularly effective when used in copper inlay-on-boronized bearings of rock bits.
U.S. Pat. No. 5,015,401, issued May 14, 1991, and assigned to the assignee of the present invention shows a rock bit bearing grease which includes a refined petroleum or hydrocarbon oil fluid base which is thickened with an alkaline metal soap or metal soap complex and which contains as solid lubricants powdered molybdenum disulfide and calcium fluoride. This grease was especially useful in carb-on-carb bearings, providing extended wear life and load carrying capacity.
Despite these advances, the lubricating greases for rock bits of the prior art have tended to use as the base or carrier fluid a refined hydrocarbon or mineral oil thickened with some type of thermally stable gelling agents, perhaps with solid lubricants or other oil soluble property enhancing additives being included, as well. Manufacturers of lubricating greases for rock bit bearings have not generally employed grease formulations with the base or carrier material being a substantial portion of a synthetic fluid or fluids. By "synthetic fluids", is meant, for example, synthetic hydrocarbon fluids or oils, polyol esters, dimer acids, synthetic polyethers and synthetic fluorinated polyethers, alkylene oxide polymers or interpolymers, esters of phosphorus containing acids, silicon based oils, or a mixture of the above type "synthetic" fluids. Commercially available base fluids of this type, such as Mobil Oil's "SHF-82", Emery Industries' "Emery 3000" and Amoco's "Polybutene Series", while utilized in, for example, the aircraft and automotive industries, have not typically been utilized in lubricating greases for rock bit bearings.
The present invention is directed toward the discovery that a grease composition suitable for use in rock bit bearings can be formulated with a synthetic fluid base and thickened with specific thickener systems to produce a grease which is particularly effective for the slow speed and highly loaded bearing configurations of rolling element and journal type rock bit bearings used to drill earthen formations.
The preferred thickener systems of the present invention impart not only gel structure to the grease but also extreme pressure and antiwear properties. The thickener systems of this invention include calcium complex soap thickeners in which calcium hydroxide and acetic acid are two of the reactants forming the thickener as well as other metal soap thickeners and their complexes in combination with calcium acetate which is either added or formed in the synthetic fluid base. The thickener systems of this invention also include non-soap thickeners such as silica gellants or clays in combination with calcium acetate which is either added or formed in the synthetic fluid base.
The lubricants of the invention have also been found to improve the performance of those rock bits which have bearing elements which are sealed from the drilling environment by a mechanical face seal. The improved lubricants in the bearing cavity of such bits functions to lubricate the bearing surface as well as functioning to effect sealing by the mechanical face seal, thereby reducing wear on the face of the seal. A reduction in seal wear and damage to the seal face is obtained on seals lubricated with the greases of the invention, as compared to the results obtained with standard rock bit greases.
A need exists, therefore, for such a bearing grease of the above type having superior lubricating properties which can be employed in lubricating the bearing surfaces of bits used for drilling in abrasive, subterranean atmospheres.
A need also exists for such a bearing grease exhibiting low wear characteristics which can be used in rock bit bearings to provide extended wear life and load carrying capacity.
A need also exists for such a grease for lubricating rock bits which has a prolonged useful life, which does not generate substantial internal pressures within the bit and which adequately protects metal bearing surfaces from premature wear or failure.
A need also exists for such a grease which improves the performance of bits having mechanical face seals beyond that obtained with currently available rock bit lubricants.